Photon Echo Decay Research Articles

Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals.

Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is crucial for the realization of efficient solar cells and light-emitting devices. We study the effect of band gap fluctuations on coherent exciton dynamics in a mixed FA0.9Cs0.1PbI2.8Br0.2 perovskite crystal by using photon echo spectroscopy. We reveal a narrow homogeneous exciton line width of 16 μeV at a temperature of 1.5 K. The corresponding exciton coherence time T2 = 83 ps is exceptionally long due to the localization of excitons at the scale of tens to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay, we conclude that for low-energy excitons pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

Microscopic dynamics of electron hopping in a semiconductor quantum well probed by spin-dependent photon echoes

Spin-dependent photon echoes in combination with pump-probe Kerr rotation are used to study the microscopic electron spin transport in a CdTe/(Cd,Mg)Te quantum well in the hopping regime. We demonstrate that independent of the particular spin relaxation mechanism, hopping of resident electrons leads to a shortening of the photon echo decay time, while the transverse spin relaxation time evaluated from pump-probe transients increases due to motional narrowing of spin dynamics in the fluctuating effective magnetic field of the lattice nuclei.

Investigation of relaxation in quantum dot ensembles in thin semiconductor films by photon-echo technique

Experimental results of observations of a femtosecond photon echo excited in an ensemble of quantum nanometer-sized objects in thin semiconductor (ZnO, SiB, SiP) films at room temperature are presented. The observed properties of the photon echo are discussed. Recorded relaxation times of exciton quantum transitions and beats at the photon echo decay as a function of the increasing interval between the exciting pulses reveal the role played by the defects of the thin film in the formation of its exciton structure of quantum transitions investigated using angular optical echo spectroscopy.

Decoherence and absorption of Er3+:KTiOPO4 (KTP) at 1.5 μm

We present results of laser absorption spectroscopy and two-pulse photon echo decoherence measurements on the lowest 4I15/2 to lowest 4I13/2 transition in Er3+: KTiOPO4 (KTP—potassium titanyl phosphate) for the optical transition located at 1537.238nm. This transition was found to have an inhomogeneous absorption linewidth of 950MHz and pronounced polarization dependence. Two-pulse photon echo decay measurements as a function of applied magnetic field strength at 1.9K revealed a narrow homogeneous linewidth of 2.5kHz at 0.2T that increased to 5.8kHz at 1.2T and then decreased to 1.6kHz at 4.5T. This behavior was successfully described by decoherence due to Er3+–Er3+ magnetic dipole interactions. Significant superhyperfine coupling of Er3+ spins to the nuclear moments of ions in the host lattice was observed, modulating the photon echo decay at low magnetic fields and limiting the effective homogenous linewidth at high fields. Combined with the well-established potential of KTP for fabrication of high-quality optical waveguides and integrated non-linear frequency conversion, our results suggest that Er3+:KTP is a promising material system for practical spectral hole burning, signal processing, and quantum information applications.

Measuring and analyzing excitation-induced decoherence in rare-earth-doped optical materials

A method is introduced for quantitatively analyzing photon echo decay measurements to characterize excitation-induced decoherence resulting from the phenomenon of instantaneous spectral diffusion. Detailed analysis is presented that allows fundamental material properties to be extracted that predict and describe excitation-induced decoherence for a broad range of measurements, applications and experimental conditions. Motivated by the need for a method that enables systematic studies of ultra-low decoherence systems and direct comparison of properties between optical materials, this approach employs simple techniques and analytical expressions that avoid the need for difficult to measure and often unknown material parameters or numerical simulations. This measurement and analysis approach is demonstrated for the 3H6 to 3H4 optical transition of three thulium-doped crystals, Tm3+:YAG, Tm3+:LiNbO3 and Tm3+:YGG, that are currently employed in quantum information and classical signal processing demonstrations where minimizing decoherence is essential to achieve high efficiencies and large signal bandwidths. These new results reveal more than two orders of magnitude variation in sensitivity to excitation-induced decoherence among the materials studied and establish that the Tm3+:YGG system offers the longest optical coherence lifetimes and the lowest levels of excitation-induced decoherence yet observed for any known thulium-doped material.

The enhanced binding energy for biexcitons in InAs quantum dots

We observed that the biexciton binding energy in InAs quantum rhombic disks (QRDs) is enhanced by twice compared with that for InAs quantum dots (QDs) so far reported around 1.24 μm nearby the telecommunication wavelength. The heterodyne-detected four-wave-mixing detected the exciton-biexciton quantum beat superposed on photon echo decay, giving the biexciton binding energy of 3.4 meV to 3 monolayer (ML) InAs QRDs and 4.1 meV to 4 ML InAs QRDs, respectively. The largest biexciton binding energy of 4.1 meV in InAs QDs is ascribed to increased electron-hole overlap in confined geometry with a minimized strain distribution.

Velocity effects in atomic and molecular collisions: Study by coherent transients

The effect of translational velocity of active atoms and molecules on the properties of photon echo is investigated using the technique of coherent transient processes. A variation in the photon-echo decay with a frequency detuning of the excitation radiation relative to the center of the vibrational-rotational transition 0 ↔ 1 ν3 R(4, 3) is observed in a mixture of 13CH3F with atomic buffers. The results are interpreted using the dependence of the echo decay rate on the magnitude of the translational velocity of active particles. The dependence of the relaxation matrix on the direction of the velocity of active atoms results in a new phenomenon of the collision-induced echo, which is investigated at the transition 0 ↔ 1 174Yb in mixtures with atomic buffers.

Coherent control of optical transient processes in a gas

The possibility of controlling the processes of the free-polarization decay and photon echo in a gas by means of resonant CW radiation has been shown. The photon echo and free-polarization decay are formed using the method of the Stark switching of levels in a low-intensity radiation field in the presence of an orthogonally polarized strong field. The experiments in the gas at the R(4, 3) transition in the vibrational band 0 ↔ 1 v313CH3F in the presence of radiation from a CW CO2 laser and the calculations by the evolution operator method show a strong effect of the high-intensity radiation field on the photon echo and free-polarization decay to its complete suppression.

Quantum Dynamics of Electronic Excitations in Biomolecular Chromophores: Role of the Protein Environment and Solvent

A biomolecular chromophore can be viewed as a quantum system with a small number of degrees of freedom interacting with an environment (the surrounding protein and solvent) which has many degrees of freedom, the majority of which can be described classically. The system-environment interaction can be described by a spectral density for a spin-boson model. The quantum dynamics of electronic excitations in the chromophore are completely determined by this spectral density, which is of great interest for describing quantum decoherence and quantum measurements. Specifically, the spectral density determines the time scale for the "collapse" of the wave function of the chromophore due to continuous measurement of its quantum state by the environment. Although of fundamental interest, there very few physical systems for which the spectral density has been determined experimentally and characterized. In contrast, here, we give the parameters for the spectral densities for a wide range of chromophores, proteins, and solvents. Expressions for the spectral density are derived for continuum dielectric models of the chromophore environment. There are contributions to the spectral density from each component of the environment: the protein, the water bound to the protein, and the bulk solvent. Each component affects the quantum dynamics of the chromophore on distinctly different time scales. Our results provide a natural description of the different time scales observed in ultrafast laser spectroscopy, including three pulse photon echo decay and dynamic Stokes shift measurements. We show that even if the chromophore is well separated from the solvent by the surrounding protein, ultrafast solvation can be still be dominated by the solvent. Consequently, we suggest that the subpicosecond solvation observed in some biomolecular chromophores should not necessarily be assigned to ultrafast protein dynamics. The magnitude of the chromophore-environment coupling is sufficiently strong that the quantum dynamics of electronic excitations in most chromophores at room temperature is incoherent, and the time scale for "collapse" of the wave function is typically less than 10 fs.

Femtosecond vibrational photon echo in CeO2-doped TeO2–Nb2O5–ZnO glass

Vibrational photon echo decay in the CeO2-doped TeO2–Nb2O5–ZnO glass is observed with near-infrared femtosecond laser pulse at the temperature of 7.5 and 300 K. The photon echo signals exhibit an initial fast decay followed by an exponential decay. The anharmonic frequency splitting (Δ), the dephasing rate (γ), the dephasing time (Td) and the linewidth (Γ) are obtained. The results show that the dephasing time of the ν=0→1 transition is dominated by the vibration state lifetime T1l, and the dephasing time of the ν=0 →2 transition is determined by both the vibration state lifetime T2l and the pure dephasing time T2d∗.

Thermal redistribution and dephasing of localized excitons in CdSe/ZnSSe quantum island structures

Temperature-dependent photoluminescence measurements of CdSe/ZnSSe quantum island structures show a blueshift of the emission maximum and a significant broadening of the emission band above 50 K. The features originate from a thermal activation of localized excitons and a redistribution of the occupation within the inhomogeneous island ensemble. The presence of exciton states with different localization at comparable transition energies is indicated by a multiexponential decay of the four-wave mixing photon echo observed at low temperatures. Upon increasing the temperature above 50 K, the decay of the four-wave mixing signal changes into a monoexponential behavior. Polarization-dependent measurements unhide strong exciton-exciton interaction and signify the formation of biexcitons with biexciton binding energies around 12 meV. The features indicate the dominance of weakly localized quantum-well-like precursor states of CdSe islands on the high-energy side of the emission band.

Unusual vibrational dynamics of the acetic acid dimer

The vibrational relaxation of the C=O stretching mode of the CH3CO2H cyclic dimer, the CH3CO2D cyclic dimer, and CH3CO2CH3 were measured in CCl4 solution at room temperature. The population relaxation of the v=1 state of the C=O mode is nonexponential, modeled with a biexponential decay having a fast time constant in the subpicosecond regime and a slow time constant of a few picoseconds. For the cyclic dimers of the acetic acids, the fast component dominates the population decay, whereas the slow component dominates the decay of the CH3CO2CH3, the model compound for the monomeric acetic acid. Deuteration of the dimer increases the relaxation time constant. The non-hydrogen-bonding monomer methyl acetate also has a subpicosecond decay constant. The pump–probe anisotropy decay reveals that the orientational dynamics of these molecules also occurs on the subpicosecond time scale and is reasonably well described by rotational diffusion in the slip hydrodynamic limit. Stimulated infrared photon echo decay experiments reveal that the correlation function of the frequency fluctuations of the cyclic acid dimer has a motionally narrowed process described by a 4 ps pure dephasing time and process with a 2.1 ps correlation time, comparable to a solvent response time. The dephasing dynamics is dominated by the population relaxation. In analyzing the photon echo data, the contribution from the rotational diffusion is incorporated by approximating the cyclic acid dimer as a symmetric top diffuser with its transition dipole located in the molecular plane but not parallel to any of the principal axes. General formulas, which will be useful in other applications, for incorporation of the diffusive dynamics of the symmetric top into the third order response functions are obtained. Nonexponential fast vibrational relaxation of C–CO2–X moiety is not adequately described by the anharmonic coupling with the nearby combination and overtone bands. In the regime where the rotational, vibrational, and dephasing times are all comparable, the solvent memory effects may play a role in vibrational dynamics, causing unusually rapid nonexponential population decay.

Echo spectroscopy of a phase-sensitive thermostat

Kinetic equations describing the behavior of an atom in nonresonant classical and broadband squeezed fields are obtained. The two-pulse photon echo under the conditions of additional illumination by non-resonant fields is studied theoretically. It is shown that investigation of the photon echo decay allows one to determine the main parameters of the squeezed field.

Heterodyne-detected accumulated photon echo in CdSe quantum dots

The homogeneous optical spectra of CdSe quantum dots were investigated under very low excitation density of 10 −3 photons per dot by using the heterodyne-detected accumulated photon echo. The photon echo decay is composed of two exponential decays in femto- and picosecond time domains. The fast decay component comes from the confined acoustic phonon sideband, while the slow one comes from the zero-phonon line. The linewidth of the zero-phonon line at 0 K is inversely proportional to the radius of the dots and ranges from 0.1 to 0.2 meV, suggesting the dominant contribution of the surface scattering. It increases linearly with the temperature rise. The temperature broadening linear coefficient is inversely proportional to the square of the radius of the dots consistence with the theory based on the deformation potential coupling to the confined acoustic phonon modes.

Nonexponential two-pulse photon echo decay in amorphous solids at low temperatures

We performed model calculations of the two-pulse photon echo decay (2PE) in amorphous solids at low temperatures in the frame of the theory of Geva and Skinner (J. Chem. Phys. 107 (1997) 7630). The nonexponential shape of the 2PE decay, predicted by this theory, makes the concept of T 2 non-unique. Thus, the measured temperature dependence of the line width depends on the definition of T 2. This problem was numerically analyzed. 2PE measurements for some amorphous systems have been performed and a strong nonexponential shape of the 2PE decay below 4 K was found.

Time-resolved four-wave mixing in InAs/InGaAs quantum-dot amplifiers under electrical injection

Time-resolved four-wave mixing in an InAs/InGaAs/GaAs electrically pumped quantum-dot amplifier is measured at room temperature for different applied bias currents going from optical absorption to gain of the device. The four-wave mixing signal from 140 fs pulses shows a transition from a delayed photon-echo response in the absorption regime to a prompt free polarization decay in the gain regime. This corresponds to a pronounced reduction of the dephasing time from 250 fs at zero bias to less than 50 fs at the maximum applied current. The four-wave mixing response at transparency of the device shows a composite structure with both photon echo and free-polarization decay. This is a signature of the digital occupation number in quantum dots, resulting at transparency in a signal from dots occupied with either zero or two excitons corresponding to absorption or gain of the dot ground state.

Down-conversion of electronic frequencies and their dephasing dynamics: Interferometric four-wave-mixing spectroscopy with broadband light

The theory for temporally multidimensional, electronically resonant four-wave-mixing (FWM) spectroscopy based on broadband light interferometry is developed. Both homodyne and heterodyne detection are considered. Upon spectrally resolving the signal, the electronic Bohr frequency is seen to be extremely down-converted, even to zero frequency, but only with homodyne detection. These very low frequency fringes, referred to as radiation difference oscillations, are well known in the interferometric coherent Raman broadband light spectroscopies. Such low frequency fringes permit interferometry in the optical region using Nyquist criteria corresponding to far infrared sampling rates. Possible applications are illustrated by simulations of the two-dimensional third order signals from an electronic two-level system in a variety of ``baths.'' It is seen how homogeneous and inhomogeneous line-shaping mechanisms manifest themselves differently and revealingly in the free induction decay, the photon echo decay and their cross term components of the FWM signal.

Biexcitonic four-wave-mixing signal in quantum wells: Photon-echo versus free-induction decay

We have performed time-integrated as well as time-resolved femtosecond degenerate four-wave-mixing measurements on inhomogeneously broadened (Zn,Cd)Se/ZnSe quantum-well structures. Potential fluctuations due to alloy disorder in the ternary wells give rise to localized quasi-zero-dimensional excitons and biexcitons. For time-integrated detection, they manifest themselves by distinct (disorder-renormalized) quantum beats. Time resolving these transients reveals a free-induction-decay-like emission nature of the biexciton in contrast to the photon-echo exciton contribution. Using a five-level scheme, containing both biexcitons and antibound two-exciton states, this striking emission-type change is explained by fluctuation and correlation effects upon the biexciton binding energy. A combination of time-resolved and time-integrated four-wave mixing is thus a powerful tool to gain information about the disorder in a given kind of quantum structure.

Theory of Single-Molecule Optical Line-Shape Distributions in Low-Temperature Glasses

We present a theoretical framework for analyzing the distribution of optical line shapes in low-temperature glasses, as measured by single-molecule spectroscopy. The theory is based on the standard tunneling two-level system model of low-temperature glasses and on the stochastic sudden jump model for the two-level system dynamics. Within this framework we present an explicit formula for the line shape of a single molecule and employ Monte Carlo simulation techniques to calculate the distribution of single-molecule line shapes. We compare our calculated line-width distributions to those measured experimentally. We find that the two-level system model captures the features of the experimental line-width distributions very well, although there are discrepancies for small line widths. We also discuss the relation of single-molecule line-shape distributions to more traditional “line shapes”, as measured by hole-burning and photon echo spectroscopies. Using the results from our analysis of the single-molecule line-width distributions, with no adjustable parameters we can compare theoretical predictions with experiment for photon echo decay times and hole widths. In general, the agreement is good, providing further evidence that the standard tunneling model in glasses is basically correct. For two systems, however, theory and experiment do not agree quantitatively.

On the Nature of Energy Transfer at Low Temperatures in the BChl a Pigment−Protein Complex of Green Sulfur Bacteria

The technique of accumulated photon echoes was used to study optical dephasing of the lowest QY transitions of the bacteriochlorophyll a complex of the green sulfur bacteria Prosthecochloris aestuarii. Variations in the photon echo decay as a function of wavelength are interpreted in terms of (phonon-assisted) downward relaxation within the QY manifold as a mechanism of energy transfer at the lowest temperatures. A temperature dependence study revealed contributions to the total dephasing both from transitions within so-called two level systems (TLS) of the host as well as activation of transitions to higher electronic levels within the QY manifold at temperatures above 5 K. Conclusions are drawn about the nature of energy transfer at low temperatures.